Consider now scattering in the physical \(t\) -channel \(14 \rightarrow 23\) . To
describe this situation we consider the center-of-mass frame and
choose helicity spinors as
follows [1]}, [2]}
\(\lambda _1 &= t^{1/4} \begin{pmatrix}1 \\0\end{pmatrix} , ~~~ \lambda _4 = t^{1/4} \begin{pmatrix}0 \\1\end{pmatrix} , \nonumber \\\lambda _2 &=i t^{1/4} \begin{pmatrix}\cos {\theta \over 2} \\\sin { \theta \over 2}\end{pmatrix}, ~~~ \lambda _3 = i t^{1/4} \begin{pmatrix}\sin {\theta \over 2} \\- \cos { \theta \over 2}\end{pmatrix} .\) 
